1. Field of the Invention
This invention relates to a heat-insulating structure of a swirl chamber in an internal combustion method and to its production method.
2. Description of the Prior Art
Generally, in combustion chambers of a swirl chamber type in an internal combustion engine, mixing of a fuel and air is made twice each in swirl chamber and main combustion chamber and the mixing state is better than in those of a direct injection type. However, the loss of cooling water is greater with the swirl chamber type than with the direct injection type and the fuel efficiency becomes lower. Therefore, attempts have been made to constitute the swirl chamber in a heat-insulating structure in order to minimize the loss of cooling water. However, in the case of the heat-insulating structure wherein the outer surface of the swirl chamber is heat-insulated uniformly, the durability problem with the swirl chamber arises due to thermal stress difference.
If the ceramic material constituting each swirl chamber block is silicon nitride (Si.sub.3 N.sub.4), silicon carbide (SiC), or the like, the ceramic material such as silicon nitride (Si.sub.3 N.sub.4), silicon carbide (SiC), or the like, has high heat resistance and can withstand a high temperature and high strength, but has high heat transfer rate and a low heat-insulating property. Since the ceramic material has a high Young's modulus and high deformation resistance, a high thermal stress acts on it if any non-uniformity occurs in its temperature distribution. Further, the temperature distribution of the inner wall surface constituting each swirl chamber is such that the jet port portion for communicating the main combustion chamber with the swirl chamber reaches a high temperature and moreover, the temperature distribution around the jet port portion is such that the jet port portion on the center side of the main combustion chamber reaches particularly a high temperature. Therefore, if each swirl chamber block constituting the inner wall portion of the swirl chamber is made of a ceramic material, the temperature distribution at the jet port portion of the swirl chamber block becomes considerably nonuniform and the thermal stress therefore acts and exerts adverse influences on the strength of the ceramic material, causing thereby the problem of durability. Accordingly, a problem remains to be solved as to how each swirl chamber itself be constituted in order to improve durability of the swirl chamber block.
A production method of a swirl chamber of an engine is known in the past from Japanese Patent Laid-Open No. 83451/1986, for example. The production method of the swirl chamber of the engine disclosed in this prior art reference fits an outer cylinder of an iron type sintered material which is subjected to compression powder molding or preparatory sintering to an inner cylinder made of a ceramic and then couples the inner and outer cylinders integrally by regular sintering to produce the swirl chamber of the engine. Namely, an insert member is prepared by integrating ceramic particles by use of a copper type bonding material and molding the integrated member in a shape substantially equal to the shape of a heat-insulating chamber to be formed at a predetermined position between the inner and outer cylinders described above, and after this insert member is interposed to the predetermined position between the inner and outer cylinders, the regular sintering step is carried out.
In the production method of the swirl chamber of the engine described above, the outer peripheral metal material consists of the sintered material. Therefore, the sintered metal has the function of only sealing the heat-insulating layer but cannot control the compressive force or the heat-insulating degree. In other words, this production method does not have the technical concept of improving durability of the inner cylinder made of the ceramic material.
An antechamber insert of an engine is described, for example, in Japanese Utility Model Laid-Open No. 173624/1985. This antechamber insert is produced by inserring a ceramic hollow member for constituting the inner wall of the antechamber of an engine into an insert metal component and is assembled into a hole that is formed in advance in a cylinder head so as to constitute the antechamber of the internal combustion engine. The ceramic hollow member described above is formed in such a manner that at least its outer peripheral surface coming into contact with the insert metal is tapered towards the open end of the ceramic hollow member.
In other words, in the antechamber insert of the engine described above, the ceramic hollow chamber for forming the swirl chamber and the portion of the jet port for communicating the main combustion chamber with the swirl chamber are constituted separately from each other. Thus, this prior art technique is not directed to control the temperature distribution for the hollow chamber and the jet port portion and to control the overall compressive force.
Furthermore, when the block constituting the swirl chamber is directly casted to a large-scale member such as the cylinder head, deviation of dimension at the time of casting becomes so excessive that the resulting product cannot be used as the product. The deviation of the casting dimension is about .+-.1.5 mm for the size of about 500 mm, for example, but accuracy of the position dimension of the swirl chamber must be about .+-.0.2 mm.
The method of imparting the residual compressive stress to the ceramic material by shrinkage fit of the metal material to the ceramic material cannot impart effective residual compressive stress because the adding direction of the compressive force is unidirectional.